In most wireless communication systems, the air link consists of the propagation channel between one transmit antenna and one receive antenna. However, it has been established that using multiple antennas at the transmitter and the receiver can significantly increase the link budget and consequently, link capacity. The drawback of this approach is that the complexity of the system can also increase dramatically. Systems with multiple transmit and receive antennas are referred to as wireless MIMO (Multi-Input Multi-Output) systems.
For MIMO systems, the increase in link budget or link capacity is achieved via one of the following approaches: increasing diversity, multiplexing, and beam-forming. When using an approach that increases diversity, similar replicas of the signal are transmitted and received by multiple antennas. These multiple transmissions are either separated (made uncorrelated) in time by using distinct delays, or in frequency by using distinct frequency offsets, or in code-space by using specific permutations and/or coding. Multiple receptions are combined using the optimal Maximal-Ratio-Combining (MRC) receiver. This approach does not require knowledge of the channel transfer function at the transmitter side. In some approaches, however, it requires significant portions of the transmit and receive data-path (analog and digital front-end) to be replicated for each antenna.
Most of the current MIMO systems follow the first (diversity) approach mentioned above. The link budget produced by this approach is roughly N times less than that resulting from beam-forming, where N is the number of antennas. Also, in most cases, the existing implementations require complex systems where entire analog and digital front-end portions of data-path are replicated per antenna. In a multiplexing scheme, accurate knowledge of the channel transfer function is used to shape the overall transmit-to-receive transfer function into separate (orthogonal) transmission links, over which data is multiplexed by using proper coding and power distribution based on the water-filling principle (more power and data over stronger links). As mentioned, this approach requires some knowledge of the channel transfer function at the transmitter side. It also requires significant portions of the transmit and receive data-path (analog and digital) to be replicated for each antenna. However, if optimally-designed, it can provide maximum capacity.
There are implementations based on the multiplexing approach, but their complexity is rather prohibitive for consumer and mobile wireless applications, unless the dimension of the MIMO system, i.e. the number of antennas, are limited, which in turn limits the maximum achievable link budget increase. In a beam-forming approach, accurate knowledge of the channel transfer function is used to focus the transmission over the strongest subspace, referred to as eigenvector, of the overall transmit-to-receive channel. The signal is then transmitted over that subspace. This is accomplished by proper adjustment of the signal phase, and possibly gain, for each transmit and receive antenna separately. This scheme clearly requires some knowledge of the channel transfer function at the transmitter side. However, it can ideally be implemented with replicating only a subset of the analog data-path, and therefore can require much simpler implementation, and/or allow a larger number of antennas to be used. It also provides better link budget than the increasing diversity approach described above and for channels that are highly correlated can approach the capacity of the multiplexing method described above. This method requires the transmission bandwidth to be a small fraction of the carrier frequency. Note that multiplexing can be accomplished via parallel beam-forming along the various eigenvectors of the transmit-to-receive channel.
Beam-forming implementations can mostly be found in radar applications, where firstly the transmitter and receiver units are the same, and secondly the objective of beam-forming is completely different from link budget or link capacity maximization. Other beam-forming proposals use direct Singular-Value Decomposition techniques that result in very complex implementations that are not suitable for consumer and mobile wireless applications, and consequently put limits on the dimension of the MIMO system, i.e. the number of antennas, and hence, the maximum achievable link budget increase.